fix(edit): unescape newString replacements to avoid literal \\n

[webwww123]

What does this PR do?

Fixes #3545.

edit currently unescapes matching for oldString but writes newString directly. Some providers send newString with escaped \\n, which can cause literal \n to be written to files.

Changes:

• move unescapeString to a shared helper in packages/opencode/src/tool/edit.ts
• apply unescape to replacement text in replace(...)
• apply unescape in the oldString === "" write path
• add regression tests in packages/opencode/test/tool/edit.test.ts

How did you verify your code works?

• Added regression tests for:
  • escaped newline replacement
  • replaceAll path
  • double-escaped \\\\n remaining literal when intended
• GitHub Actions passed on this PR:
  • typecheck
  • unit (linux)
  • e2e (linux)
  • e2e (windows)
  • test (linux)

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found

[github-actions]

Closing this pull request because it has had no updates for more than 60 days. If you plan to continue working on it, feel free to reopen or open a new PR.